Combustion of fossil fuels typically produces an exhaust gas stream (commonly referred to as a “flue gas stream”) that contains contaminants, such as carbon dioxide (CO2), sulfur oxides (SOx), nitrogen oxides (NOx), mercury, and carbon containing species, as well as particulate matter such as dust or fly ash. To meet requirements established under certain laws and protocols, plants that burn fossil fuels subject the resultant flue gas stream to various processes and systems to reduce or eliminate the amount of contaminants present in the flue gas stream prior to releasing the flue gas stream to the atmosphere.
In one example, carbon dioxide is removed from a gas stream by introducing the gas stream to an absorber column (“absorber”) in a counter current flow with a solvent. Contact between the solvent and the gas stream allows the solvent to absorb and thus remove the carbon dioxide from the gas stream. The gas stream that is free of the carbon dioxide may be further processed while the carbon dioxide rich solvent is regenerated for further use in the absorber tower.
When using reactive solvents, such as amines, ammonia, ionic liquids, alkali carbonates, etc., for carbon dioxide removal, the solution reactions between the solvents and the carbon dioxide are usually exothermic. Exothermic heat of reaction cannot be eliminated from the carbon dioxide removal process and efficient carbon dioxide removal is inherently accompanied by an exothermic reaction. Even though the amount of heat generated in the removal of the carbon dioxide is equal to the heat which must be added to the regenerator for breaking the bonds and freeing the carbon dioxide from the solvent, much of the heat generated by the exothermic reaction is lost, for example, by being carried out of the absorber by the gas stream. Additionally, the increase in the temperature within the solvent acts to reduce the equilibrium solubility of carbon dioxide, and hence, reduces removal capacity in the absorber. Interstage cooling has been employed to minimize these effects.
A disadvantage of known methods and devices that have been used to reduce the temperature in the absorber and the energy used in the absorption and regeneration system is that these methods often require reconfiguration of the absorber, expenditure of capital, and plant shut-down. Additionally, previously developed and used systems have not taken full advantage of the heat energy created in the absorber.